A pyrolysis or similar unit for the production of ethylene yields a cracked gas which is fractionally condensed in one or more stages. Each stage produces an overhead vapor and a liquid fraction. The overhead vapors from each stage are directed to the subsequent stages, while the liquid fractions are directed to other processing units. For example, the liquid fractions may be fed to a multi-feed demethanizer tower which produces a liquid having large proportions of ethylene and an overhead vapor having large proportions of methane and a smaller amount of residual ethylene.
The overhead vapor from the last stage of the fractional condensers is a gas that is rich in hydrogen and methane and contains significant amounts of ethylene. This overhead vapor from the last stage of the fractional condensers has been used to cool other parts of this or related systems. In other applications, the overhead vapor from the last stage has been further separated into its main components of hydrogen and methane. Both of these optional treatments for the overhead vapor are referred to, for example, in U.S. Pat. No. 4,002,042.
The prior art systems of the type described above have been deficient in certain respects. First, the ethylene included in the overhead vapor from the last stage of the fractional condensers is for all practical purposes wasted. Consequently, the recovery of ethylene by the total system is lower than desired. Second, the ethylene in the overhead vapor from the last stage of the fractional condensers affects the purity of the hydrogen obtained in any subsequent separation steps. To obtain a purer quality of hydrogen requires more complex equipment which operates at an increased cost. Additionally, in some systems, steps that provide purer hydrogen result in a lower yield of hydrogen.